The present invention relates to a sequence controller, especially a sequence controller in which a malfunction caused by electrical noises introduced from the outside is prevented.
Generally, a sequence controller performs a prescribed sequence process following a sequence program which is set beforehand based on the input data from a control object (external load) showing the external condition, and the process result is output to the control object through an output portion. An electronic circuit, such as an arithmetic circuit, which constitutes a sequence controller is generally constituted with a TTL circuit, and the signal level is low in the order of 5 VDC. On the other hand, the power supply voltage for signals of a control object such as an external output load or an external input system is as high as 24 VDC or 100/200 VAC, so that when an external signal is turned ON or OFF, a still higher surge voltage (1 kV to 2 kV) can be induced.
To prevent an electronic circuit of a low voltage level from being subjected to a malfunction effected by a noise voltage of a high level, a control object (external load) and the arithmetic circuit (electronic circuit) of the sequence controller are connected to an insulating element, such as a photocoupler. Both of these circuits are optically coupled for transmitting a process result and also for preventing noise transmission in the direction of the flow of an ordinary signal by electrical insulation.
The technique mentioned above is an effective method for preventing a malfunction of an electronic circuit by cutting off electric noises from the control objects, but a surge voltage from a noise generating source is applied to the wirings of control objects through stray couplings in various parts. If the noise voltage is a continuous high frequency pulse-formed high voltage (generally called a burst noise), such as a noise generated when an AC magnet-switch is turned off, it travels through an insulating element through stray coupling. In an output portion, especially, the noise from a control object is applied directly to the output side of the insulating element in a direction opposite to that of an ordinary signal flow, which causes a malfunction in the electronic circuit.
Because of this, a malfunction can be caused by noise in the output circuit following the insulation element even though a correct process result is transmitted from an arithmetic circuit.